Huckleberry friend

Hi everybody!
There is an expression “huckleberry friend” that means “friend indeed” or about it. But why “huckleberry”? From what this expression originated? Thanks.

Most believed that it is from the friendship of Tom Sawyer and his friend Huckleberry Finn. However, this phrase comes from the song, "Moon River", the theme to "Breakfast at Tiffany". The lyricist, Johnny Mercer described in his autobiography, said that it was in reference to a childhood friend of his. He used to pick huckleberries with him down by a lazy river near his home in Georgia.  So it is the berry and not the friend in the book, "Tom Sawyer".